1. Field of the Invention
This invention relates to digital radiographic systems and more particularly to a process for correcting digital image values from a defective pixel column.
2. Description of Related Art
In the past decade there has been great progress made in the area of direct radiographic imaging using detectors comprising a two dimensional array of minute sensors to capture a radiation generated image. Information representing an image is captured, often as a charge distribution stored in a plurality of charge storage capacitors in individual sensors arrayed in a two dimensional matrix. We will refer to such detectors generically as direct radiographic detectors, or simply as detectors, to differentiate them from the often referred to traditional radiographic detectors which employ a photosensitive film usually in combination with an intensifying screen to produce a photographic image of the incident X-ray radiation.
The direct radiographic detectors typically comprise a two dimensional array of sensors with associated switching and addressing circuitry built on an insulating substrate, usually a glass plate. U.S. Pat. No. 5,319,206 issued to Lee et al. on Jun. 7, 1997, shows a typical direct radiation detector comprising an array of sensors for the generation and capture of charges following exposure to X-ray radiation. Readout of the stored charges is accomplished in any one of a plurality of manners. U.S. Pat. No. 5,648,660, also by Lee et al. discloses a method for the readout of stored charges in a direct radiographic imaging panel.
Direct radiation detectors offer a number of distinct advantages over the traditional film methods. The availability of a radiogram in electronic signal format, permits the use of digital signal conversion and all the advantages of signal storing, retrieval, transmission and processing associated with digital imaging.
Direct radiation detectors, however, are not free of problems uniquely associated with them. Practical diagnostic quality and size detectors require panels comprising millions of individual sensors arrayed in rows and columns. Typically, such sensors are scanned to obtain the electrical signal stored therein which represents the radiogram. Scanning is accomplished through a process that uses a plurality of access switches associated with the individual detectors. Switching is usually done with an FET transistor by addressing its gate and the signal is recovered through a source line connected to the FET source electrode.
Any defective sensor or line will result in loss or distortion of the signal from the sensor or from a plurality of sensors, resulting in what is commonly referred to as Bad pixel error. Bad pixel errors produce image artifacts which interfere with the ability to properly read the radiogram and may produce false readings.
There are presently a number of ways known to identify and compensate for the presence of bad pixels in imaging panels comprising a plurality of sensors. It is known for instance to obtain a flat field exposure of the detector (that is a uniform intensity exposure) and read out the resulting image. In a perfect detector, the signal output level from each sensor would be exactly the same. In reality, the output varies somewhat from sensor to sensor. Variations within acceptable limits are devised and sensors whose output falls outside predetermined acceptable limits are defined as "bad" pixels. These bad pixels are typically mapped and their output replaced by a calculated value computed by interpolation from a plurality of neighboring pixels. Variations in the output of the acceptable pixels are compensated by a pixel gain adjustment for each sensor so that the final output is substantially uniform for the flat field exposure.
While the above approach to solving the bad pixel problem is useful in correcting individual bad pixels, there is another type of problem that remains unsolved. At times there is a column of a plurality of sensors all connected to the same source addressing conductor (line) that is defective. When this occurs, it has been observed that the output of a number of adjacent sensor columns is also effected, and, furthermore, that the effect on the adjacent sensors is not linear with exposure. (Exposure is the product of the radiation intensity incident on the detector multiplied by the time the radiation impacts the detector.)
This problem produces undesirable and unpredictable artifacts which cannot be adequately corrected by the known methods. There is, therefore need to provide a method for correcting such bad pixel column defects.